Bridge scour

Bridge scour is the removal of sediment such as sand and rocks from around bridgeabutments or piers. Scour, caused by swiftly moving water, can scoop out scour holes, compromising the integrity of a structure.[1]

In the United States of America, bridge scour is one of the three main causes of bridge failure (the others being collision and overloading). It has been estimated that 60% of all bridge failures result from scour and other hydraulic-related causes.[2] It is the most common cause of highway bridge failure in the United States,[3] where 46 of 86 major bridge failures resulted from scour near piers from 1961 to 1976.[4]

Water normally flows faster around piers and abutments making them susceptible to local scour. At bridge openings, contraction scour can occur when water accelerates as it flows through an opening that is narrower than the channel upstream from the bridge. Degradation scour occurs both upstream and downstream from a bridge over large areas. Over long periods of time, this can result in lowering of the stream bed.[2]

Stream channel instability resulting in river erosion and changing angles-of-attack can contribute to bridge scour. Debris can also have a substantial impact on bridge scour in several ways. A build-up of material can reduce the size of the waterway under a bridge causing contraction scour in the channel. A build-up of debris on the abutment can increase the obstruction area and increase local scour. Debris can deflect the water flow, changing the angle of attack, increasing local scour. Debris might also shift the entire channel around the bridge causing increased water flow and scour in another location.[3]

During flooding, although the foundations of a bridge might not suffer damage, the fill behind abutments may scour. This type of damage typically occurs with single-span bridges with vertical wall abutments.

The examination process is normally conducted by hydrologists and hydrologic technicians, and involves a review of historical engineering information about the bridge, followed by a visual inspection. Information is recorded about the type of rock or sediment carried by the river, and the angle at which the river flows toward and away from the bridge. The area under the bridge is also inspected for holes and other evidence of scour.

Bridge examination begins by office investigation. The history of the bridge and any previous scour related problems should be noted. Once a bridge is recognized as a potential scour bridge, it will proceed to further evaluation including field review, scour vulnerability analysis and prioritizing. Bridges will also be rated in different categories and prioritized for scour risk. Future implementation of scour countermeasures includes monitoring plans, inspections after flood events, and procedures for closing bridges if necessary.

Alternatively, sensing technologies are also being put in place for scour assessment. The scour-sensing level can be classified into three levels: general bridge inspection, collecting limited data and collecting detailed data.[5] There are three different types of scour-monitoring systems: fixed, portable and geophysical positioning. Each system can help to detect scour damage in an effort to avoid bridge failure, thus increasing public safety.

Bend way weirs, spurs and guide banks can help to align the upstream flow while riprap, gabions, articulated concrete blocks and grout filled mattresses can mechanically stabilize the pier and abutment slopes.[6]Riprap remains the most common countermeasure used to prevent scour at bridge abutments. A number of physical additions to the abutments of bridges can help prevent scour, such as the installation of gabions and stone pitching upstream from the foundation. The addition of sheet piles or interlocking prefabricated concrete blocks can also offer protection. These countermeasures do not change the scouring flow and are temporary since the components are known to move or be washed away in a flood.[7] FHWA recommends design criteria in HEC-18 and 23, such as avoiding unfavourable flow patterns, streamlining the abutments, and designing pier foundations resistant to scour without depending upon the use of riprap or other countermeasures.

Trapezoidal-shaped channels through a bridge can significantly decrease local scour depths compared to vertical wall abutments, as they provide a smoother transition through a bridge opening. This eliminates abrupt corners that cause turbulent areas. Spur dikes, barbs, groynes, and vanes are river training structures that change stream hydraulics to mitigate undesirable erosion or deposits. They are usually used on unstable stream channels to help redirect stream flow to more desirable locations through the bridge. The insertion of piles or deeper footings is also used to help strengthen bridges.

Hydraulic Engineering Circular No. 18 Manual (HEC-18) was published by the Federal Highway Administration (FHWA). This manual includes several techniques of estimating scour depth. The empirical scour equations for live bed scour, clear water scour, and local scour at piers and abutment are shown in Chapter 5-General Scour section. The total scour depth is determined by adding three scour components which includes the long-term aggradation and degradation of the river bed, general scour at the bridge and local scour at the piers or abutment.[8] However, research had shown that the standard equations in HEC-18 over-predict scour depth for a number of hydraulic and geologic conditions. Most of the HEC-18 relationships are based on laboratory flume studies conducted with sand-sized sediments increased with factors of safety that are not easily recognizable or adjustable.[9] Sand and fine gravel are the most easily eroded bed materials, but streams frequently contain much more scour resistant materials such as compact till, stiff clay, and shale. The consequences of using design methods based on a single soil type are especially significant for many major physiographic provinces with distinctly different geologic conditions and foundation materials. This can lead to overly conservative design values for scour in low risk or non-critical hydrologic conditions. Thus, equation improvements are continued to be made in an effort to minimize the underestimation and overestimation of scour.

Thornton-Tomasetti, P. C. (1987) “Overview Report Investigation of the New York State Thruway Schoharie Creek Bridge Collapse.” Prepared for: New York State Disaster Preparedness Commission, December 1987.